Field of the Invention
The present invention relates to an amplifier circuit.
Output stages are realized using bipolar transistors in radio frequency technology, for example.
For transistors, various limiting data which must not be exceeded are specified, such as, for example in the case of bipolar transistors, the emitter-base reverse voltage, the maximum permissible collector-emitter voltage and also the collector-base reverse voltage. By way of example, if the maximum permissible collector-emitter voltage is exceeded on account of a high supply voltage, then undesirable breakdown effects occur. In this case, significantly larger currents flow with a reduced output power. These breakdown effects have the disadvantage that the breakdown, as soon as it is initiated, no longer cuts off.
Consequently, there is a requirement that the operating voltage must lie below the maximum permissible collector-emitter voltage in every operating state. If the operating voltage is prescribed, for example by the battery voltage of a portable device, and may vary for example depending on the charged state of the battery, then it is necessary to upgrade the amplifier circuit for higher supply voltages.
It is conceivable, for example, to use power transistors with a higher collector-emitter breakdown voltage. However, these have the disadvantage of requiring complex process modifications in production and can only be realized with a significantly larger chip area in conjunction with a lower performance, in particular lower efficiency.
U.S. Pat. No. 4,353,037 specifies a protection circuit for a transmitter amplifier, wherein the gain of the amplifier is regulated in order to avoid mismatches thereof.
It is accordingly an object of the invention to provide an amplifier circuit that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which is suitable for higher supply voltages, in which the breakdown effects outlined are avoided, and which can be realized using standard transistors without an increased maximum permissible collector-emitter voltage UCE0.
With the foregoing and other objects in view there is provided, in accordance with the invention, an amplifier circuit containing a reference-ground potential terminal and an amplifier stage having an input for receiving a signal to be amplified, an output at which an amplified signal can be drawn off, and a reference node. An in-phase regulator having an input side for receiving a regulating voltage, a load side having a first end connected to the reference node and a second end connected to the reference-ground potential terminal, is provided. The in-phase regulator reduces a DC voltage dropped across the amplifier stage between the output and the reference node. A voltage regulator is provided and has a supply potential terminal for receiving a supply potential and an output outputting the regulating voltage received by the in-phase regulator. The regulating voltage has a fixed potential difference with respect to the supply potential.
According to the invention, the in-phase regulator is provided on the reference-ground potential side, that is to say on the ground side on an amplifier stage, for example on a radio frequency output stage. As a result of this, depending on the regulating voltage, the voltage that is dropped across the amplifier stage and depends on the supply voltage can be reduced, with the result that no breakdown effects occur in transistors of the amplifier circuit. Since the voltage dropped across the transistors of the amplifier stage is reduced, high-voltage bipolar or high-voltage CMOS technology is not required, with the result that the amplifier circuit described can be produced with little complexity, a small area requirement and cost-effectively. Nevertheless, there is a large modulation range available for the operating frequency of the amplifier stage, which may be a high frequency. This may be governed at least partly by parasitic transistor capacitances.
Furthermore, in accordance with the present principle, a voltage regulator is provided, which is connected to a supply potential terminal and provides the regulating voltage at its output, the regulating voltage having a fixed potential difference with respect to the supply potential. The voltage regulator forms a fixed potential difference between the supply potential, which also supplies the amplifier stage, and the potential at the reference node. The voltage regulator can consequently be used to set the maximum voltage dropped across the amplifier stage, with the result that it is thereby ensured that a voltage that is higher than the maximum permissible collector-emitter voltage is not dropped across the amplifier stage. In contrast, if the supply voltage is lower than the fixedly set potential difference, which is a fixed maximum potential difference, then the controllable resistor requires a low resistance and draws the reference node practically to reference-ground potential in DC voltage terms.
Consequently, the present circuit limits the voltage dropped across the output stage of an amplifier to a maximum value.
In one advantageous embodiment of the present invention, the in-phase regulator has a resistor, which can be controlled by the regulating voltage, and is connected on the one hand to the reference node and on the other hand to the reference-ground potential terminal. This has the advantage of power-free control.
In a further advantageous embodiment of the present invention, the controllable resistor is a MOS transistor. N-channel transistors, in particular, make it possible to realize the in-phase regulator with a low residual voltage, i.e. that at a low supply voltage of the amplifier stage, only a very low voltage magnitude is dropped across the load path of the MOS transistor, the load path having a low impedance in this case. On account of parasitic capacitances in the transistor, the reference node is virtually at reference-ground potential for the operating frequency even at a high supply voltage. The power-free driving of the MOS transistor makes it possible to use a regulator circuit with a low current demand.
In a further advantageous embodiment of the present invention, a regulating loop is formed in the in-phase regulator, the regulating loop containing a first operational amplifier, to which the regulator voltage can be fed at its inverting input and which is connected to the reference node by its non-inverting input, an output of the first operational amplifier being coupled to a control input of the controllable resistor for the control of the latter. The regulating loop described realizes a voltage follower in which the potential at the reference node follows the potential at the inverting input of the first operational amplifier, i.e. the regulating voltage.
In a further advantageous embodiment of the present invention, a loop filter is provided between the output of the first operational amplifier and the control input of the controllable resistor. The loop filter enables stable operation of the regulating loop with a sufficient phase margin even when the first operational amplifier is a non-ideal operational amplifier.
In a further advantageous embodiment of the present invention, for the purpose of voltage regulation, a second operational amplifier is provided, to which a reference signal can be fed at its non-inverting input, whose output is fed back to its inverting input, and whose inputs are coupled to the supply potential terminal. A regulating voltage that is fixedly related to the supply potential can be provided in a simple manner by the second operational amplifier.
In a further advantageous embodiment of the present invention, for the purpose of voltage regulation, a further controllable resistor is provided, whose control input is connected to the output of the second operational amplifier and via whose controlled path, which is connected to the reference-ground potential terminal at one end, the regulating voltage can be drawn off.
If the controllable resistor is a MOS transistor, for example, then it is possible to operate the voltage regulation with a very small residual voltage, with the result that, at a low supply voltage, the regulating voltage potential which can be fed to the inverting input of the first operational amplifier can be reduced to virtually reference-ground potential.
In a further advantageous embodiment of the present invention, a resonant circuit is connected between the reference node and the reference-ground potential terminal. The resonant circuit provides improved linking of the reference node with low impedance to the reference-ground potential for the operating frequency of the amplifier stage. The amplifying properties of the amplifier circuit can be improved as a result of this.
In a further advantageous embodiment of the present invention, the resonant circuit has an inductance in series with a capacitance. In this case, the capacitance can be produced in a simple manner by an external capacitor which is connected to the reference node via a bonding wire which realizes the inductance.
In accordance with an added feature of the invention, an inductive transformer is provided and connected to the input of the amplifier stage. The inductive transformer converts a common-mode signal present on an input side into a single-ended signal for driving the amplifier stage.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an amplifier circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.